The present invention relates to a solid-state image sensing device, and more particularly, to a CCD (charge coupled device) solid-state image sensing device employing a frame transfer system.
FIG. 1 is a schematic block diagram of a conventional CCD solid-state image sensing device 100 employing a frame transfer system. The CCD solid-state image sensing device 100 includes an image sensing part 102, a storage part 104, a horizontal transfer part 106 and an output part 108. The image sensing part 102 comprises a plurality of shift-registers (not shown) that extend in the vertical direction and are disposed parallel with each other. The shift-registers each comprise a plurality of register bits that correspond to light-receiving pixels. The storage part 104 comprises a plurality of light-blocked shift-registers (not shown) disposed adjoining the shift-registers of the image sensing part 102. Each of the storage part shift-registers comprises a plurality of register bits that form storage pixels. The horizontal transfer part 106 comprises a single shift-register (not shown) disposed extending in the horizontal direction. The transfer part shift-register comprises a plurality of shift-register bits connected with a plurality of shift-register outputs of the storage part 104, respectively. The output part 108 comprises capacitors for temporarily storing charges transferred from the horizontal transfer part 106 and reset-transistors for discharging the capacitors.
The light-receiving pixel register bits of the image sensing part 102 are transferred to the storage pixels of the storage part 104. The storage pixel bits stored in the storage part 104 are then transferred to the horizontal transfer part 106 in a unit of one line. The pixel data is then transferred from the horizontal transfer part 106 to the output part 108 in a unit of one pixel. A voltage that corresponds to each of the register bits is generated by the output part 108 and is output from the CCD solid-state image sensing device 100.
FIG. 2 is a schematic plan view of the image sensing part 102 which is a three phase driving type. FIG. 3A is a cross-sectional view along line 3Axe2x80x943A of FIG. 2 and FIG. 3B is a cross-sectional view along line 3Bxe2x80x943B of FIG. 2.
A device area formed of a P-type diffusion region 2 is formed on a principal surface of an N-type silicon substrate 1. A plurality of thick channel isolation regions 3 formed by selective oxidation are disposed parallel with each other on the surface of the diffusion region 2. A plurality of N-type diffusion regions 4 are disposed between each of the channel isolation regions 3. The diffusion regions 4 are channel regions that are used as transfer paths by pixel data bits. Thin gate insulation films 5, which preferably comprise silicon dioxide films, are disposed integrally with the channel isolation regions 3 on the diffusion regions 4. A plurality of transfer electrodes 6, which preferably comprise polycrystalline silicon, are disposed parallel with each other in a direction that is perpendicular to the channel isolation regions 3. The transfer electrodes 6 are separated from each other by a constant distance on the gate insulation films 5 and the channel isolation regions 3. In order to control the potential of the channel region (diffusion regions 4), for example, in the case of three electrodes making one set, three phase transfer clocks xcfx861-xcfx863 are applied to the transfer electrodes 6. An interlayer insulation film 7 having the same composition as the gate insulation films 5 covers the transfer electrodes 6.
In a solid-state image sensing device employing a frame transfer system, light-receiving pixels are formed in the channel regions covered by the transfer electrodes 6. Since the light-receiving pixels are covered by the transfer electrodes 6, light reaches a photoelectric transfer area (the channel region) through the transfer electrodes 6. Generally, polycrystalline silicon, which forms the transfer electrodes 6, has high absorptance and reflectance for shorter wavelength light. Therefore, polycrystalline silicon passes little light of shorter wavelength and thus decreases the amount of such shorter wavelength light reaching the light-receiving pixels. This deteriorates the sensitivity of the light-receiving pixels for blue light (for example light having a wavelength from 4000 xc3x85to 5000 xc3x85).
Accordingly, it is an objective of the present invention to provide a solid-state image sensing device that includes light-receiving pixels that have an improved sensitivity to shorter wavelength light.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a solid state image sensing device is provided. The device includes a semiconductor substrate, a gate insulation film and a plurality of transfer electrodes. The semiconductor substrate has a plurality of channel regions extending in a first direction. The channel regions extend substantially parallel to each other with a predetermined spacing therebetween. The gate insulation film is located on a part of the semiconductor substrate that includes the channel regions. The transfer electrodes are formed on the gate insulation film and extend substantially parallel to each other in a second direction to intersect with the channel regions. Each transfer electrode includes a polycrystalline silicon layer disposed on the gate insulating film and a silicon nitride layer disposed on the polycrystalline silicon layer.
The present invention may also be applied to a method of manufacturing a solid state image sensing device. The method includes the steps of: forming a plurality of isolation regions extending substantially parallel to each other on a semiconductor substrate and forming a channel region between adjacent isolation regions; depositing a gate insulation film on the semiconductor substrate for covering the channel regions; depositing a polycrystalline silicon layer and a silicon nitride layer on the gate insulation film and patterning the polycrystalline silicon layer and the silicon nitride layer for forming a plurality of transfer electrodes intersecting with the isolation regions; and forming an inter-layer insulation film covering the transfer electrodes.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.